JP2015109789A - Power generation system and method with fault ride through capability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power generation system and method with fault ride through capability.SOLUTION: A power generation system 40 includes a generator 42 mechanically coupled to an engine 60 to generate electrical power, and a fault ride through system connected between the generator and a power grid 44. The fault ride through system 46 includes a mechanical switch 54 connected in parallel with a solid state switch 52 and a resistor 53 to absorb power from the generator during a grid fault condition. The mechanical switch and the solid state switch are controlled in coordination with the engine.

Description

  The present invention relates generally to electrical energy conversion, and more particularly to a system and method for a fall rid-through function of a small generator set connected to a power grid with a low moment of inertia.

  In conventional power systems, most of the power is generated in large-scale centralized facilities such as fossil fuel power plants (coal, gas power generation), nuclear power plants, or hydroelectric power plants. Although these conventional power plants have excellent economies of scale, they usually transmit electricity over long distances and can affect the environment. A distributed energy resource (DER) system is a small generator set (typically in the range of 3 kilowatts to 10,000 kilowatts) used to provide an alternative or extension to conventional power systems. The small generator set may be generated by a gas engine, a diesel engine or a wind turbine, for example. Since DER systems are generated very close to where electricity is used, they reduce the amount of energy lost when transmitting power. The DER system also reduces the size and number of power lines that must be installed. However, due to the increasing trend towards distributed generation using small generator sets, many grid cords require small generator sets to provide enhanced features such as fault voltage ride-through.

  When a fault occurs in the power system, the voltage in the system can drop in a significant amount for a short period of time (typically less than 500 milliseconds) before the fault is eliminated. The fault may be caused by at least one phase conductor (ground fault) connected to ground or a short circuit of two or more phase conductors. These types of faults can occur during lightning strikes, wind storms, or transmission lines connected to ground due to accidents. Faults can result in serious voltage drop events. In the past, under these inadvertent failures and large-scale power failure situations, small generator sets have been allowed and desired to trip offline whenever a voltage drop occurs. Such an operation has no actual detrimental effect on the power supply when the penetration rate of small generator sets is low. However, as the proliferation of small generators within the power system increases, these small generators remain online and can continue to supply power to the power grid after the fault is removed. It is desirable to ride through such low voltage conditions and maintain synchronization with the power grid. This is similar to the requirements that apply to large generator sets.

  Therefore, it is desirable to determine a method and system that addresses the aforementioned problems.

US Patent Application Publication No. 2012/0175876

  According to an embodiment of the present technology, a power generation system is provided. The power generation system includes a generator mechanically coupled to the engine to generate power. The power generation system also includes a fault ride through system connected between the generator and the power grid. The fault ride through system includes a solid state switch that absorbs power from the generator during a network fault condition and a mechanical switch connected in parallel with the resistor. In the power generation system, the mechanical switch and the solid state switch are controlled in cooperation with the engine.

  According to another embodiment of the present technology, a method for supplying power from a power generation system to a power grid is provided. The power generation system is connected between the generator and the power grid and includes a fall rid through system including a resistor connected in parallel with the mechanical switch and the solid state switch. The method includes controlling to open the mechanical switch when a fault is detected, and to close the mechanical switch when the fault is eliminated after a predetermined time. The method also provides a bypass path for generator current via a solid-state switch or resistor after the mechanical switch is opened and before a predetermined time, and works with the solid-state switch for engine ignition. Including controlling.

  These and other features, aspects and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 4 is a plot of a grid profile with voltage profiles immediately before, during and immediately after a fault. 2 is a schematic representation of the use of a power generation system connected to a power grid and a fall-through-through system according to aspects of the present disclosure. 2 is a schematic representation of certain stages of a fall-through operation according to aspects of the present disclosure. 2 is a schematic representation of certain stages of a fall-through operation according to aspects of the present disclosure. 2 is a schematic representation of certain stages of a fall-through operation according to aspects of the present disclosure. 2 is a schematic representation of certain stages of a fall-through operation according to aspects of the present disclosure. 2 is a schematic representation of certain stages of a fall-through operation according to aspects of the present disclosure. 2 is a schematic representation of certain stages of a fall-through operation according to aspects of the present disclosure.

  As will be described in detail below, embodiments of the present invention function to provide a system and method for a fall rid-through function of a small generator set connected to a power grid with a low moment of inertia.

  FIG. 1 shows a plot 10 of an example grid code voltage profile at a point of connection (POC) to a generator power grid. Some of the network authorities expect that the generator should not be disconnected from the network if the voltage at the POC is higher than the voltage profile shown. However, this is one exemplary case, and voltage profile requirements may vary from country to country or from network authorities. Plot 10 shows a horizontal axis 12 representing time in milliseconds and a vertical axis 14 representing voltage in percentage of nominal voltage. The failure occurs in 0 milliseconds. Before the failure, the system is in a stable state, so the pre-failure voltage at POC 16 is 100% or 1 per unit before 0 milliseconds. Due to a fault in the network, the voltage 18 at 0 milliseconds drops to only 5% at the beginning of the fault. It should be noted that the voltage drop at the POC depends on the distance of the fault to the POC, fault impedance, fault type, network characteristics, etc. In one embodiment, the voltage may be less than 5%, or in another embodiment, the voltage may be greater than 5%.

  When the voltage drops to a level as shown in FIG. 1, the generator may not be able to export full power to the network during low voltage conditions. At the same time, if the prime mover continues to provide constant mechanical power to the generator, this will result in acceleration of the engine generator's rotational mass and the rotor speed will increase. Increasing the rotor speed will result in an excessive increase in the rotor angle of the synchronous generator, which can result in loss of synchronization. Therefore, the generator will trip and will not meet the grid code requirements. In some countries, grid code requirements are stringent and generators may need to ride through longer fault times. According to one embodiment of the present technology, a fault voltage ride-through system using solid state switches in combination with resistance and engine control is disclosed to address the aforementioned problems.

  FIG. 2 illustrates a power generation system 40 connected to a power grid 44 that utilizes a fall rid through system 46 according to an embodiment of the present invention. The power generation system 40 includes a generator 42 connected to a prime mover 60 and a power grid 44. In one embodiment, the generator 42 has a small rated power, for example less than 10 MW. Further, the generator is mechanically coupled to a prime mover 60, which can be a turbine or an engine. In one embodiment, engine 60 comprises a gas turbine or gas engine or wind turbine. In some embodiments, the generator 42 will be coupled to the power grid 44 via a power converter (not shown), and in other embodiments the generator 42 may be connected to the power grid 44 without any power converter. Will be coupled to. The generator 42 may be connected to the power grid 44, the transformer 48, and the transmission line 50 via a fault ride through system 46. It should be noted that the configuration shown in FIG. 2 is for illustrative purposes only, and in another embodiment, the fault laid through system 46 may be connected between the transformer 48 and the power grid 44. . It should be noted that FIG. 2 shows a single line diagram of the electrical system for ease of explanation. The fault ride through system 46 includes a solid state switch 52, a resistor 53, a mechanical switch 54 and a controller 56. The component, solid state switch 52, resistor 53 and mechanical switch 54 are all connected in parallel to each other, while the fall rid through system 46 is connected in series with the generator 42. In one embodiment, the solid state switch 52 may comprise an integrated gate rectifying thyristor (IGCT), an insulated gate bipolar transistor (IGBT), or an alternating current triode (TRIAC). Controller 56 receives one or more input signals 58 and provides control signals to solid state switch 52, mechanical switch 54 and engine 60. In one embodiment, the input signal 58 comprises any of a voltage signal, a current signal, a generator power signal, a speed signal, a rotor angle signal, an engine output signal, an engine torque signal, or any combination thereof. The controller uses the input signal 58 to determine whether a failure has occurred on the system, and controls to control the operation of the engine 60, solid state switch 52 and mechanical switch 54 in the event of a failure. Provide a signal.

  In operation, solid state switch 52 is non-conductive or off during normal conditions, while mechanical switch 54 is conductive or on. When there is a fault in the network, the voltage at the generator connection point (POC) 62 drops significantly. If the low voltage condition at the POC continues for a threshold time, the generator 42 may be exposed to very high currents due to the large angle between the generator rotor and the mesh. The generator will therefore disconnect from the net to protect itself from these high currents. The growth angle between the generator rotor and the net may also lead to loss of synchronization between the generator and the net, and it will be necessary to disconnect the generator from the net. However, to meet the grid code fall-through-through requirement, after the fault is eliminated and the voltage at the POC is restored to the pre-failure level, the generator maintains the connection to the network and supplies power to the network. You should be able to continue. In other words, the generator speed and rotor angle should remain within acceptable limits as long as the voltage at the POC is above the voltage profile given by the grid code during a fault condition.

  The voltage drop at the POC due to a fault event in the power grid is detected by the controller 56 and the generator 42 can be decelerated from acceleration or stopped because the power that the generator can supply to the network during the low voltage condition at the POC is limited. To trigger the engine 60 connected to the generator 42 to reduce power (eg, partially or completely switch off the engine ignition). In certain cases, stable operation of the gas engine is possible only if the ignition is not turned off longer than a certain period of time (eg, one or more engine cycles) before being turned on again. . A fall rid-through system 46 according to embodiments of the present technology provides for such power generation where the duration of the low voltage condition that the generator must ride through is longer than the maximum time that the engine ignition can be held off. The machine set can meet grid code requirements.

  In one embodiment, almost simultaneously when the controller 56 triggers the engine 60 to reduce power due to a fault, the mechanical switch 54 is also triggered to switch off and the solid state switch 52 is also triggered to switch on. As an example, solid state switch 52 can be switched on in a few microseconds, but mechanical switch 54 is switched off in the millisecond range (eg, between 30 milliseconds and 100 milliseconds) after triggering. Will.

  Once the mechanical switch 54 is fully off, the total current is redirected through the solid state switch 52 due to its negligible on resistance compared to the resistor 53. When the engine ignition is turned on again, the controller 56 begins regulating the current flowing through the resistor 53 by controlling the current flowing through the solid state switch 52. This method results in an effective resistance adjustment of the resistor 53 in series with the generator 42. By changing the effective value of the resistor 53 in series with the generator 42, the acceleration, speed and rotor angle of the generator during the failure can be adjusted. In other words, the generator current is redirected to the solid state switch and resistor during a fault condition and after the mechanical switch is opened. Further, the current through resistor 53 is adjusted by controlling the current through solid state switch 52, thereby consuming part or all of the engine power in resistor 53.

  If the fault is eliminated within a given time and the voltage at the POC is back to an acceptable level at which the generator can power the network, the engine ignition is switched back on and still partially Or when it is completely off, the mechanical switch 54 is triggered to be switched on. At the same time, the solid state switch 52 can either be placed in the ON position to short the resistor 53 by way of example, or continue to be controlled by the controller 56 to adjust the generator speed or rotor angle. Eventually, when the mechanical switch 54 is in the on state, the solid state switch 52 is placed in the off position by the controller 56 to return the fall-through through system 46 to its initial state during the normal state prior to the failure event. However, if the fault is not eliminated within a given time and the voltage at the POC is below the voltage profile given by the grid code, then the engine is triggered to turn off completely and disconnected from the power grid 44. As a result, no power is supplied to the power grid 44 by the generator 42 in the end.

The active power consumed by resistor 53 in the event of a fault depends on the voltage across the resistor and is generally given by Vr ² / R, where Vr is the root mean square (RMS) voltage across the resistor and R Is the resistance value of the resistor. As described above, when Vr is 0.3 pu and R is 0.1 pu, the power consumed by the variable resistor that is assumed to be in the OFF state of the solid state switch 52 is supplied by the generator It will be 0.9 pu, which is almost equivalent to the total power being done. In other words, resistor 53 in this case can consume up to 90% of the power supplied to the generator by the engine, thereby significantly reducing the generator's acceleration during low voltage conditions. In this way, the generator can maintain its rotational speed or rotor angle within an acceptable range and does not need to be disconnected from the network during or after a failure.

  FIGS. 3 (a) through 3 (f) illustrate various stages of a fall rid through operation according to aspects of the present disclosure. FIG. 3A shows a normal state or a fault-free state (t <0) only when the mechanical switch 54 is conductive and the solid-state switch 52 is not conductive. During this phase, the generator current 70 flows only through the mechanical switch 54 and not through the solid state switch 52. Since the mechanical switch 54 shorts the resistor 53, no current flows through the resistor 53. At t = 0 (FIG. 3 (b)), a failure event occurs in the network, and at t = 20 milliseconds (FIG. 3 (c)), the failure event is detected by the fall rid through system. In one embodiment, a fault event may be detected when the voltage falls below a specified low value for a specified time. In other embodiments, the fault event may be detected based on a voltage signal, current signal, speed signal, power signal, torque signal or rotor angle signal, or any combination thereof. As can be seen in FIGS. 3 (b) and 3 (c), during these phases, the generator current 70 still flows through the mechanical switch 54 because no control action has been initiated. Note that the timing shown here (ie, t = 0, 20, 120 milliseconds, etc.) is for illustrative purposes only, and in other embodiments, the timing may be based on system and control parameters. Should. In addition, when a fault event is detected by the fault ride through system at t = 20 milliseconds, a first control signal is sent to the generator engine to partially or fully switch its ignition off. At the same time or after a while, a second control signal is sent to the mechanical switch 54 to open it. Since the switch-on time of the solid-state switch 52 is much shorter than the switch-off time of the mechanical switch 54, the third control signal that switches on the solid-state switch 52 is also at t = 20 milliseconds or some It may be transmitted with a delay. However, the solid state switch 52 should be switched on well before the mechanical switch 54 is completely switched off. During this time, generator current 70 may flow through both solid state switch 52 and mechanical switch 54.

  FIG. 3 (d) shows that at t = 120 milliseconds, mechanical switch 54 is completely switched off and solid state switch 52 is switched on. In one embodiment, the engine ignition is also commanded to switch on after the mechanical switch 54 is fully open. This is typically done in embodiments where engine ignition cannot be switched off for longer periods (eg, one or more engine cycles). In one embodiment, the conduction time of the solid state switch 52 is controlled. In other words, the current through the solid state switch 52 is controlled and the remaining generator current 70 flows through the resistor 53. Therefore, the amount of power consumed by the resistor 53 can be controlled by controlling the solid state switch. The purpose of the controller 56 is to adjust the generator speed to the nominal synchronous speed, as an example. This is accomplished by changing the amount of “braking force” consumed in the resistor 53. Another control purpose of the controller 56 may be to adjust the rotor angle of the synchronous generator or the power supplied to the network at the POC.

  If the fault is eliminated before a predetermined time, eg t = 250 milliseconds, and if it is still partially or completely switched off, the controller switches the ignition fully on to the engine. A first control signal is transmitted and at the same time a second control signal is transmitted to trigger the mechanical switch 54 to close it. The controller 56 continues to control the solid state switch 52 until the mechanical switch 54 is fully closed, for example at t = 551 milliseconds. The solid state switch 52 is then switched off, and the normal operating state before the failure event is restored as shown in FIG.

  If the fault is still not eliminated at a given time, eg t = 250 milliseconds, or if the voltage at the POC is below the voltage profile given by the grid code, the engine will trigger to switch off completely Is done. The solid state switch 52 is triggered to switch off simultaneously or after some time (eg, after the generator has stopped rotating), and the current 70 flows through only the resistor 53 as shown in FIG. The power is consumed completely or partially in resistance until the engine 60 is finally switched off and disconnected from the net.

  While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. Accordingly, it is to be understood that the claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

10 Plot of exemplary grid code voltage profile 12 Horizontal axis 14 Vertical axis 16 Pre-failure voltage 18 Fault-time voltage 40 Power generation system 42 Generator 44 Power grid 46 Fault ride-through system 48 Transformer 50 Transmission line 52 Solid state switch 53 Resistance 54 Machine Type switch 56 controller 58 input signal 60 prime mover / engine 62 connection point (POC)
70 Generator current

Claims (20)

A power generation system (40),
A generator (42) mechanically coupled to the engine (60) for generating electric power;
A fall fold through system (46) connected between the generator (42) and a power grid (44), wherein the fold ride through system (46) from the generator (42) during a network fault condition. A fall-through-through system (46) comprising a solid state switch (52) for absorbing power and a mechanical switch (54) connected in parallel with a resistor (53);
With
The mechanical switch (54) and the solid state switch (52) are controlled in cooperation with the engine (60).
Power generation system (40).   The power generation system (40) of claim 1, wherein the engine (60) comprises a gas turbine or a gas engine or a wind turbine.   A first control signal for controlling the mechanical switch (54) when the network fault condition is detected, a second control signal for controlling an ignition of the engine (60), and the solid state switch The power generation system (40) of claim 1, further comprising a controller (56) that generates a third control signal for controlling (52).   The controller (56) switches off the mechanical switch (54) when the network fault is detected, and switches on the mechanical switch (54) when the network fault is eliminated after a predetermined time. The power generation system (40) of claim 3, wherein the power generation system (40) is configured to:   The power generation system (40) of claim 4, wherein the controller (56) is configured to switch off the solid state switch (52) when the fault condition is eliminated after the predetermined time period.   The power generation system of claim 4, wherein the controller (56) is configured to partially or completely switch off the ignition of the engine for a specified time when the network fault is detected. (40).   The controller (56) disconnects the mechanical switch (54) and the solid state switch (52) when the fault condition is not eliminated in the predetermined time and the generator (42) is shut down. The power generation system (40) of claim 6, configured to be held in a state.   The controller (56) is configured to control the solid state switch (52) to adjust the speed or rotor angle of the generator (42) by changing the current flowing through the resistor (53). The power generation system (40) according to claim 3.   The power generation system (40) of claim 3, wherein the controller (56) is configured to detect the fault condition based on an input signal (58).   The power generation system (40) of claim 9, wherein the controller (56) controls the solid state switch (52) and the engine (60) based on the input signal (58).   The power generation of claim 10, wherein the input signal (58) comprises any of a voltage signal, a current signal, a generator power signal, a speed signal, a rotor angle signal, an engine output signal, a torque signal, or any combination thereof. System (40).   The power generation system (40) of claim 1, wherein the solid state switch (52) comprises an integrated gate rectifying thyristor (IGCT), an insulated gate bipolar transistor (IGBT) or an alternating current triode (TRIAC). A method of supplying power to a power grid (44) from a power generation system (40) comprising a fall rid through system (46) connected between a generator (42) and a power grid (44), the fault tide The slew system (46) comprises a solid state switch (52) and a resistor (53) connected in parallel with a mechanical switch (54),
Controlling the mechanical switch (54) to open when a fault is detected, and closing the mechanical switch (54) when the fault is eliminated after a predetermined time; ,
Provide a bypass path for generator current (70) through the solid state switch (52) or the resistor (53) after the mechanical switch (54) is opened and before the predetermined time period. Steps,
Controlling the ignition of an engine coupled to the generator (42) in cooperation with the solid state switch (52);
A method comprising:   14. The method of claim 13, further comprising partially or completely switching off the ignition of the engine (60) for a specified time when the fault is detected.   Further comprising switching off the mechanical switch (54) and the solid state switch (52) when the fault is not eliminated after the predetermined time and the generator (42) is shut down. Item 14. The method according to Item 13.   The method of claim 13, further comprising controlling a solid state switch (52) to adjust a speed or rotor angle of the generator (42) by changing a current flowing through the resistor (53). .   The method of claim 13, wherein the fault is detected based on an input signal (58).   The method of claim 17, wherein the solid state switch (52) and the engine (60) are controlled based on the input signal (58).   The method of claim 18, wherein the input signal (58) comprises any of a voltage signal, a current signal, a generator power signal, a speed signal, a rotor angle signal, an engine output signal, a torque signal, or any combination thereof. .   14. The method of claim 13, wherein the solid state switch (52) comprises an integrated gate rectifying thyristor (IGCT), an insulated gate bipolar transistor (IGBT) or an alternating current triode (TRIAC).